Varnish Removal and Control in Turbine Lubrication Systems

Varnish deposits on metal surfaces in turbine lube system results in a number of adverse operational issues, especially the restriction and sticking of moving mechanical parts such as servo or directional control valves. The oil has limited solvency for the material, hence a typical turbine will have the majority of the material as deposits and a relatively small portion as suspended material in the oil phase in quasi-equilibrium with the deposits. The lube system needs to be cleaned by removing the suspended varnish precursors from the oil phase, which allows the deposits to re-entrain into the oil phase, until the majority of the transferable deposits from internal surfaces are removed and the oil carries no significant amount of the material to have any adverse effect. The methods used for the removal of varnish from turbine lube oil systems include chemical cleaning - flushing, and electrostatic charge induced agglomeration - retention and the adsorption of the oil suspended varnish on an adsorbent medium. The paper discusses an absorption based removal method that utilizes a fibrous medium that has pronounced affinity for the removal and retention of the varnish forming material from the oil and the deposits from surfaces that are in quasi-equilibrium with the varnish precursors in the oil. The filtration medium is composite cellulose with specially formulated, temperature cured binder resins. The absorptive medium that exhibits high structural and chemical integrity has been thoroughly tested on operating turbines, showing reduction in varnish levels from critical range to below normal range in a relatively short time. The experiences with the utilization of the absorptive medium in laboratory tests and in two operating turbines are presented.